Arno Penzias, and Robert Wilson were two scientists who were working with AT&T helping develop satellite communications in 1964.
They stumbled onto microwave radio waves at 160 Gigahertz and they could not figure out where it was coming from as it was uniform wherever they aimed their antennas. They finally realized this was radiation left over from “The Big Bang.”
When you put 160 Gigahertz in Planck’s constant, you can solve for the average temperature of the Universe.
It is 3o Kelvin.